Production of the soft magnetic sheets used in various electronic devices is usually carried out by a kneading and rolling method. In such a method, a flat soft magnetic powder, a rubber, and a binder such as chlorinated polyethylene are kneaded in certain proportions by a kneader. The obtained kneaded mixture is rolled to a certain thickness with an apparatus such as a calendar roll. The binder may then optionally be heated to cause crosslinking, whereby a single-layer soft magnetic sheet is obtained. This method has the advantages that the soft magnetic powder can be filled to a high density, the soft magnetic powder can be oriented in an in-plane direction by the rolling, and adjustment of the sheet thickness is easy.
However, in the case of a kneading and rolling method, since strain occurs in the soft magnetic powder during the kneading, the magnetic properties of the soft magnetic powder itself deteriorate, so that there is the problem that the magnetic permeability of the soft magnetic sheet cannot be increased. Furthermore, the sheet thickness becomes thicker under a high temperature or high-temperature, high humidity environment, so that there is the problem that magnetic permeability decreases.
Therefore, instead of a kneading and rolling method, soft magnetic sheets may be produced by a coating method in which strain does not easily occur in the soft magnetic powder (Patent Document 1). In this method, a liquid composition for forming a soft magnetic sheet composed of a flat soft magnetic powder, a rubber, a resin, and a solvent, is applied to a release substrate. Then, by drying the liquid composition with which the release substrate has been coated, a soft magnetic sheet is obtained which has a small sheet thickness change even under a high temperature or high-temperature, high humidity environment.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-243615